1. Field of the Invention
The present invention relates to a laser processing apparatus, and particularly to a processing apparatus for cutting and welding relatively thick steel plates and the like.
2. Description of the Prior Art
A laser beam can be used for cutting, welding, marking and the like of materials including ferrous metal such as steel plates and nonferrous metals such as aluminum, and for cutting and carving synthetic resin materials including acryl board and the like. The most commonly used laser oscillator has a comparatively small output, so that only the processing of a laminate can be performed. Recently, however, a laser oscillator having a more powerful output has been disclosed, so that processing of comparatively thick plates has become practicable.
For example, when a steel plate is to be cut using a laser, a condensing lens is provided for the laser beam and a torch is provided for jetting oxygen as an assistant gas. A spot is formed on the surface of the plate by condensing the laser-light with the lens, and the steel plate is cut by moving the light along a previously established cutting line while an oxygen gas is jetted against the laser spot. The cutting is enforced by continuously melting and evaporating the steel plate-basic material in accordance with the energy of the laser-light, and a sudden oxidation of the molten basic material occurs as a result of the oxygen gas so that removal of the oxidized product from the steel plate can be effected.
The laser-light radiated from the laser oscillator is not parallel and has a slightly extensive angle. Accordingly, when the distance between the laser oscillator and the torch is changed, the diameter of the spot which is formed on the steel plate is changed and cutting becomes inefficient. Therefore, in the laser processing apparatus for enforcing secondary cutting, welding, sculpture and the like, the condensing position of the laser-light toward the surface of the working material must be maintained at a fixed place so that the condensing light diameter (spot-diameter) of the laser-light must be maintained constant.
Generally, when cutting, welding and the like are performed using a laser beam, excellent efficiency is obtained by irradiating the laser-light to the working material in a thin, condensed state. However, as a result of work done by the present inventor, it became clear that the cutting of a thick plate cannot be performed satisfactorily using only the laser-light in a thin beam. In theory, when the laser light is irradiated to the steel plate and oxygen gas is jetted at the same time, energy is concentrated at the irradiation point of the laser-light, and melting and evaporation of the basic material can occur suddenly to a small extent. Thus, it is possible that combustion of the basic material can begin at this point. However, it is possible also that heating of the basic material may still be insufficient for melting and combustion.